5.9 How to harvest energy with nano-power DC/DC solutions

Hi, and welcome to this training video-- How to Harvest Enough Energy from the Source to Power the Load with Nano-Power DC-DC solutions. We're going to discuss two energy harvesting techniques to power your solution.
Nano-power applications can best make use of energy harvesting as their power source. Such applications include smart home, smart thermostats, smart locks, smart doors, smart windows, as well as the fitness bands and sports watches and activity trackers that we all wear. As well, hard-to-reach applications can really benefit from energy harvesting. Instead of sending out a technician every month or every year to change a battery, these applications can run without a battery and last forever.
Things such as implantable devices inside the human body or sensors on pipelines are good examples. Finally, environmental awareness, where the sensor is measuring the temperature, pressure, or humidity of a system and sending that information to a central host to make decisions about reliability, process variation, and so on.
These designs are all united by low data rates and low duty cycles, and can therefore run on nano average powers. As such, they are suitable for energy harvesting. All around us, every day, there is abundant energy to harvest and make use of. Sources like light, EM waves, vibrations on bridges, and just the heat generated by our bodies are all sources of energy that can be used for other purposes.
So which of these sources are most effective? Which can be used? This table shows the approximate output power of various sources, such as light, thermal, vibration, and RF energy. As we can see, light produces by far the most power per unit area. This is probably one reason why it's the most common energy harvesting source in the market today.
On the bottom is also shown the typical average power consumptions of various devices. As we can see, there's still quite a bit of gap on the higher power devices to the amount of power output possible from the harvesters.
So this becomes our challenge to solve. How can we harvest enough energy from a particular source to power a particular load? Nano-power DC-DC converters are the solution.
The first energy harvesting example we will look at today is an RF switch. This is a light switch in your home. On the drawing here in the red is the switch part the user will push. We have a magnet inside with the two poles. And this is creating a magnet field through this core back to the other pole.
When the user flips the switch, you can see the field changes. It goes the opposite direction through the core. And we know from Maxwell's equations that this change in magnetic field through these windings will produce a current. This is energy that we can harvest and make use of.
Now, the end goal, of course, is to turn on a light without having to run a wire between the switch and light fixture. This must be done wirelessly. Looking at the switch for an instant, we have the generator, which I showed in the previous slide. It outputs an AC voltage which goes both above and below ground.
This is not useful for electronics, so we rectify this through a diode to make a positive voltage. Then we put this energy into a capacitor to make this voltage waveform. This is still not useful for our radio at the end of this, so we must add DC-DC converter, a buck converter, to take this varying input voltage and make a well-regulated-- 1.8-volt, for example-- voltage to power the radio. Now our system can take the harvested energy created by the user's action at the light switch and turn on the light.
Now, what sort of DC-DC converter can you use to take this energy stored in the capacitor and create the regulated 1.8 volt for your radio? Well, the TPS 6212 family, with the TPS 62125 in particular, is an excellent fit, because it operates from a wide input voltage range up to 15 or 17 volts and consumes very little quiescent current or IQ, around 11 microamps.
Now, this is important because this is the overhead current required to operate just the DC-DC converter. It's the current we throw away just to begin converting the power. And we don't want to throw away too much of our harvested energy, s a low IQ is important.
The 62125 also has a very small input and output capacitor to give a small solution size, as well as it has a special feature, a window comparator, or an input supervisor. This tells the DC-DC converter when to turn on and when to turn off.
And such a window comparator, or SVS, is important in such applications to provide a reliable and clean startup. With the TPS 62125, the turn on and turn off voltages are fully programmable. So as the input voltage ramps up after the light switch has been moved, we get to the threshold where the DC-DC converter turns on. And here's where the output voltage starts to ramp up.
Now, there's always some inrush current drawn as we start up the converter, any converter. And this will drag down the input voltage, because we have not harvested all the energy by this point. This can cause issues with the proper startup of the DC-DC and of the output voltage.
But having enough energy-- this much energy-- stored in that capacitor enables us to have a clean startup. Once we start up, the energy demands are fairly low by the radio, and so the input voltage continues rising as we keep accumulating the harvest energy. Once we use the energy, the DC-DC turns off at a predetermined location and a clean shutdown is observed.
Now, here's a real RF switch and the waveforms we will see from this. The green is the output of the harvester. Then that voltage is taken into the blue, into the capacitor. And the pink here is the output of the DC-DC converter, well regulated for the entire 20-millisecond time.
So during those 20 milliseconds, we have to start up the radio and transmit to turn on the switch, and then shut down properly. And we can do that in the 20 milliseconds.
Now to a more familiar energy harvesting application with solar. How can we use solar to transmit via radio? Well, we have our solar cells over here taking that sunlight. And we put them in series to create a higher voltage.
Of course, it's usually not enough energy in the cells themselves to complete a task, so we have to store that energy somewhere, and a capacitor is an easy choice. Then we take a nano-power DC-DC converter, the 62740, to create the core rail needed for the CC430 radio.
And this one has a built-in ADC, analog to digital converter, to sense the voltage of a solar panels. Now, this is important to make sure that we have enough energy stored in that capacitor to start up and perform the required tasks of the system.
And we've implemented such a solution in our solar device, a wireless sensor node, in the internet of things. Now, this dice has six solar panels, one on each side, and they're connected in series. And whenever the dice is rolled, the onboard accelerometer wakes up. The orientation, which side is up, is transmitted to the host.
Now, the entire thing draws a couple microamps when it's in standby. And this is due to ultra low power-optimized power supply and ultra low power-optimized code. The MCU has to be very intelligent about when it wakes up based on how much stored energy there is in that capacitor. And can it wake up, take the measurement with the ADC, and transmit that to the host before it runs out of energy?
Now, this is an example of a wireless sensor node. You can measure anything with this-- temperature, pressure, humidity, and so on. All the design files are found on our reference design page, PMP 9754.
Thank you for listening to this training today. Here's a summary of the devices and reference designs we've talked about, with more information, including our entire portfolio, available at ti.com/dcdc. Thank you.

Description

June 10, 2016

This training video looks at two specific nano-power, energy harvesting solutions, an RF switch, and the Solar Dice, to understand the requirements of the power management that enables them. The RF Switch is a wireless light switch which harvests its energy from the user’s action of the switch itself. The Solar Dice uses solar cells to harvest enough energy to measure its orientation (which side is up) and then wirelessly transmit this back to a host. Both are examples of building automation equipment that enable the Internet of Things (IoT).